(57) In printing apparatus including means (464,465) for changing a value to be printed
and means (470) for selecting a value to be printed, wherein the value changing means
includes a plurality of banks (460,464), each of the banks includes a print wheel
(464) having a plurality of print elements (465), and wherein the value selection
means includes means (472) for selecting each bank and means (476,480) for selecting
each print element (465) of a selected bank, and means (120) for driving the bank
and print element selection means, wherein the driving means includes an output shaft
(122), and means for selectively coupling the output shaft to the bank and print element
selection means (470), an improvement for controlling the value selection means, the
improvement comprising: the driving means including a d.c. motor (120) having the
output shaft (122); means (126) for sensing angular displacement of the motor output
shaft; a computer (500) comprising clock means for generating successive sampling
time periods, means for providing first counts respectively representative of successive
desired angular displacements of the motor output shaft (122) during successive sampling
time periods, means (270) responsive to the sensing means (126) for providing second
counts respectively representative of actual angular displacements of the motor output
shaft (122) during successive sampling time periods, and means for compensating for
the difference between the first and second counts during each successive sampling
time period and generating a pulse width modulated control signal for controlling
the d.c. motor (120), the motor control signal causing the actual angular displacement
of the motor output shaft (122) to substantially match the desired angular displacement
of the motor output shaft during successive sampling time periods; and signal amplifying
means (300) for operably coupling the motor control signal to the d.c. motor (120).
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